One of the barriers to publication/disclosure of useful data sets is that it is a low priority compared other policy or service delivery obligations. The reality is that agency heads prioritise activities towards those things that are critical to delivery of their outputs and that are reported on in annual reports.
Publication and disclosure of data requires effort (to ensure that the data is 'clean', accurately described by metadata and in accessible formats etc.) and creates risks (arising from erroneous interpretation, conflicting data sources and exposure of things that are inconvenient for the Government in terms of the control of policy discussions etc.) As a consequence we shouldn't be surprised that agency heads regard publication of data as a relatively low priority compared to many other more pressing operational responsibilities. It is a 'nice to have', not an essential activity – and is a risk that is perhaps best avoided.
This will be unlikely to change unless publication of data sets to acknowledge as an 'official' function of the agency. One way to do this would be for each agency to be required to identify a catalogue of data sets to be made available to the public and for publication of these to be included in the agency's formal outputs. Annual reports would then include an analysis of the agency's performance at publishing the agreed data sets.
This would make routine publication of an agreed core catalogue of useful and relevant data sets a mandatory activity - rather than something that is done on a voluntary and piecemeal basis.